(a) Field of the Invention
This invention concerns gas seals and in particular relates to a sealing member for and a method of effecting a gas seal between a shaft and a housing which surrounds with clearance the shaft, as well as to a complete seal assembly comprising a rotatable shaft, a housing surrounding the shaft and a sealing member fitted therebetween.
(b) Description of the Prior Art
There have been designed various forms of seal capable of effecting a seal between a rotatable shaft and a housing which surrounds the shaft, for instance to prevent foreign matter which may be present outside the housing on one side of the seal entering the housing. The provision of an effective seal is particularly important for the case where a bearing for the shaft is provided within the housing, to prevent damage to the bearing, and in such a case, the seal may serve the further purpose of preventing lubricant or other liquids from leaving the housing.
A known form of seal suitable for the above purposes comprises a member which is adapted to be mounted in the housing and fits around the shaft, the member for instance being made of a resilient material such as a natural or synthetic elastomer. Such a seal member is quite satisfactory for effecting a seal to a shaft intended to rotate at low speeds, but as the speed of rotation rises, so does the rate of wear of both the seal member and shaft itself. Consequently, with shafts intended to rotate at very high speeds (for instance, 10,000 r.p.m. or more), it is advantageous to provide a seal member which is retained within the housing in such a way that the member does not contact the shaft but fits very closely therearound with a minimal clearance. For example, the seal member may be elongate axially and provided with a plurality of circular ribs on its inner surface, which ribs fit within but do not touch correspondingly shaped grooves formed in the shaft; such a seal is known as a labyrinth seal. The sealing effect of a labyrinth seal may be enhanced by providing air or a gas under pressure at an appropriate point to the seal member, so that the air or gas bleeds outwardly through the labyrinth path defined by the shaft and the seal member, thereby reducing the probability of foreign matter entering the labyrinth path.
The above-described form of labyrinth seal suffers from several disadvantages. The seal member must be made in at least two parts in order to be capable of being fitted around the shaft, and this causes problems in manufacture and assembly. Machining of the seal member is critical and must be formed to tight tolerances; machining of the grooves in the shaft may also cause problems. The seal member must be mounted rigidly within the housing to ensure that the member does not contact the shaft at any point, and moreover if clearances are to be maintained at the smallest values possible, then the shaft must run with a very high degree of truth; little if any run-out is permissible. Also the supply of compressed air or gas to the seal leads to further complications and an increase in the amount of equipment necessary.